1. Field of the Invention
The present invention relates to a solid-state image sensor and image sensing apparatus and, more specifically, to a wiring layout of the solid-state image sensor.
2. Description of the Related Art
A general image sensing apparatus such as a digital camera or digital video camera uses a solid-state image sensor such as a CCD image sensor or CMOS image sensor. In a majority of these solid-state image sensors, a plurality of wiring layers of circuitries, wirings, signal read units, and the like are formed on an Si substrate on which a photoelectric conversion layer arranged with elements such as photodiodes as photoelectric conversion elements, which configure pixels, is formed. The plurality of wiring layers are isolated from each other by interlayer films, the uppermost surface is planarized, color filters are formed on the uppermost layer, and microlenses ML used to collect light are formed on the color filters. The solid-state image sensor is irradiated with light coming from an optical system of, for example, a lens connected to an image sensing apparatus from the surface side on which the microlenses are formed.
For the solid-state image sensor, light coming from the optical system of the image sensing apparatus via the microlenses and color filters reaches the photodiodes via the wiring layers. In a route of the incoming light before the photodiodes, if the incoming light is blocked off and reflected by metal wirings of the wiring layers, various problems can occur that cause a drop in image quality. For example, a problem of a sensitivity drop is caused when some incoming light rays are blocked off by some wirings and do not reach the photodiodes. Also, a problem of color mixture that lowers color reproducibility is posed when some light rays reflected by metal wirings become incident on pixels different from those which the rays originally enter to result in poor color separation.
These problems caused by interception and reflection of light by the wiring layers are conspicuous especially in a CMOS image sensor, which requires a plurality of wiring layers due to a complicated wiring layout, of the solid-state image sensors.
As an image sensor which solves the aforementioned problems, a solid-state image sensor called a backside illumination type has been put into practical applications in CMOS image sensors.
In a conventional solid-state image sensor, wiring layers are arranged above color filters, microlenses, and photodiodes, while in the backside illumination solid-state image sensor, wiring layers are formed below photodiodes (for example, FIG. 3 of Japanese Patent Laid-Open No. 2003-31785). With this arrangement, light which has passed through the microlenses and color filters enters the photodiodes without going through the wiring layers, thus solving problems of interception and reflection of light by the wiring layer.
Furthermore, since the wiring layers are not located between the light incident position and the photodiodes, openings required to allow incoming light to enter the photodiodes need not be formed in regions where the wiring layers overlap the photodiodes unlike in the conventional image sensor. Thus, metal wirings on the wiring layers can be laid out without any limitations of these regions, thus enhancing degrees of freedom in wiring layout, and facilitating the complicated wiring layout. This is also a merit of the backside illumination solid-state image sensor. Using this merit, in the commercially available backside illumination solid-state image sensor, metal wirings on the wiring layers are laid out at free positions without forming any openings depending on the positions of the photodiodes, as described in Japanese Patent Laid-Open No. 2003-31785 (FIG. 6).
In the backside illumination solid-state image sensor disclosed in Japanese Patent Laid-Open No. 2003-31785 above, since light enters from a side opposite to the wiring layer side, incoming light can be prevented from being blocked off or reflected by the metal wirings before it reaches the photodiodes, thus solving these problems.
However, since the wiring layers are arranged at positions deeper than the photoelectric conversion layer when viewed from the incident side of light, a new problem is posed. More specifically, light which has once passed through the photoelectric conversion layer is reflected by the wiring layer arranged at the position deeper than the photoelectric conversion layer, and enters the photodiodes of the photoelectric conversion layer again, resulting in a drop in color reproducibility.
The drop in color reproducibility caused by reflected light which enters the photodiodes will be described below. The Si substrate used in the solid-state image sensor has a wavelength dependence on light absorption coefficients, and photoelectric conversion depths in the photodiodes formed on the Si substrate are different depending on wavelengths of incoming light. More specifically, light on the short wavelength side, which light is closer to blue, is photoelectrically converted at a relatively shallow position in the Si substrate, and light on the long wavelength side, which light is closer to red, is photoelectrically converted at a relatively deep position in the Si substrate. Infrared (IR) light is photoelectrically converted while entering to a still deeper position in the Si substrate than the red light. That is, light closer to IR light tends to enter a deeper part in the Si substrate compared to light closer to blue.
For this reason, light of a given wavelength, which has entered the photoelectric conversion layer but is not photoelectrically converted by the photodiodes on the photoelectric conversion layer, passes through the photoelectric conversion layer including the photodiodes, reaches the wiring layer, and is reflected by the metal wirings of the wiring layers. The reflected light strikes the photoelectric conversion layer including the photodiodes again, and is photoelectrically converted by the struck photodiode if that photodiode has sensitivity to a wavelength of the light.
Si (silicon) absorbs light rays up to a wavelength of about 1,100 nm. That is, as is known, Si has sensitivity to light rays up to the wavelength of about 1,100 nm. On the other hand, human eyes have sensitivity to light rays up to a wavelength of about 780 nm. That is, signals obtained by the solid-state image sensor unwantedly include light rays outside a wavelength range to which the human eyes are sensitive.
For a general image sensing apparatus used to shoot photos and videos, reproducibility of a visible object is important. Hence, it is desired that such image sensing apparatus has sensitivity to only visible light of human. If the solid-state image sensor equipped in such image sensing apparatus has sensitivity to IR light, a color reproducibility drop occurs, thus disturbing to shoot preferable photos.
As a countermeasure to be taken against such problems, in a general digital camera, an IR cut filter as one type of optical filters is arranged between the solid-state image sensor and optical lens, thereby preventing the solid-state image sensor from being irradiated with IR light. For example, if an ideal IR cut filter, which can perfectly block off IR light over a full wavelength range, is used, the IR light can be prevented from reaching the solid-state image sensor. However, when use of such ideal IR cut filter results in an increase in component cost of an image sensing apparatus, it is not practical to adopt such filter in an image sensing apparatus which is to be provided to general consumers with as lower price as possible.
Therefore, a digital camera for general users achieves a cost reduction by using an IR cut filter with a limited wavelength range that can be blocked off. More specifically, for example, such IR cut filter blocks off IR light rays within a wavelength range of about 700 nm to 1,000 nm to prevent the solid-state image sensor from being irradiated with these IR light rays. However, since that IR cut filter cannot block off light rays in a wavelength range of about 1,000 nm or higher in terms of its characteristics, some light rays pass through the IR cut filter and enter the solid-state image sensor. Of light rays in the wavelength range of about 1,000 nm or higher, which have reached the solid-state image sensor, those in a wavelength range of about 1,100 nm or lower, to which Si has sensitivity, are photoelectrically converted by the photodiodes. That is, signals obtained by the solid-state image sensor unwantedly include light rays outside the wavelength range to which the human eyes are sensitive.